Saturday, September 30, 2017

UMMD was built to print decorative objects like large vases and lamps. A couple failed test prints, including this one at the Milwaukee Maker Faire:

Looks great, doesn't it?

It's too bad the back side wasn't so great.

That taught me a lesson: you can't print a single-walled, ABS vase with 0.4 mm line width and 0.2 mm layers that is 500 mm tall. Between the cooling plastic shrinking and the weight of the print distorting its shape, the nozzle will eventually miss the previous layer and the print will fail.

I switched to a larger nozzle, adjusted the slicing parameters a bit and produced this:

This one made it all the way to 500 mm with only a couple minor issues.

This was made using transparent ABS which looks like frosted glass when it prints, and transmits light very nicely. I used a 0.6 mm nozzle on the extruder, printed in 0.3 mm layers, 0.6 mm line widths, and printed with 3 shells/perimeters, all at 60 mm/sec. It's 500 mm tall, took about 39 hours to print and used 781 g of filament.

I'm not sure why the slicer has the extruder going all over the place like that, so there may be more tweaking to do, but this one is definitely a success.

I still have to mount it on some sort of base, and add a light source, but here's what it looks like with an LED flashlight lighting it up from the inside:

Unlike a single walled vase, this thing can be handled without worrying about it breaking apart.

When the vase was removed from the print bed, the bottom layer had a couple small cracks, possibly because the 95C bed temperature was a little higher than it should have been for almost 40 hours. I'll drop the bed temperature a little more for the next one.

There are a couple small layer separations on the back side which may have been because the temperature inside the printer was a bit too cool for ABS. ABS is usually OK with 45-50C but during this print the temperature in the enclosure was only about 38C. When I drop the bed temperature, it's going to be even cooler inside the enclosure, so I'll be adding a heater to make it a little warmer in there.

This could be printed with PLA and the layer separation issues would probably go away, but I have to make sure that the print is never subjected to heat, either from a light source or from being transported and left in a hot vehicle.

How I created the model

I started with a program called ChaosPro to generate a Julia set fractal. After tweaking the parameters for a while I found a shape that I liked, then created an image series that varied one of the parameters of the fractal over a specified range of values. That left me with 500 or so images.

Next, I opened ImageJ and used it to stack the images to make a solid object from them, then exported the STL file of that solid.

I liked the rough texture that resulted from the process, so I skipped the smoothing that the guy did using Blender.

I have tried to print this model using Slic3r's spiral vase mode but it seems to choke on the STL file (maybe the surface is too rough in some places and slic3r can't follow it) and does strange things that wreck the print. I've been using Cura to slice it, and Cura applies some sort of minor smoothing that leaves most of the rough texture intact but fixes the problems that trip Slic3r.

Thursday, September 28, 2017

Here's a project I did about 11 years ago, years before I built my first 3D printer. It's a Van De Graaff generator (VDG) that produces about 400 kV (that's enough to thrown painful sparks about 300 mm in dry air!).

I never liked the look of the wood box on the bottom, and it was all a little heavy, so a few months ago I decided to update the design.

I redesigned the base and rollers to be 3D printable and found a small DC motor that could be mounted on the base without the big wood box. The rollers use bearings pulled from hard disk drives. I printed the parts using PLA.

I took it to the Milwaukee Maker Faire last week just to show what can be done with a 3D printer, and after it sat unnoticed for a few hours, decided to move it closer to foot traffic and plug it in. If you ever want to attract kids to a booth at a product show or Maker Faire, just bring along a VDG! As these people demonstrated a Van De Graaff generator can be a lot of fun!

I suspect this was Kylee's favorite thing at the Maker Faire. She spent a lot of time with us!

In the photos below I used a 30" exposure time and high ISO, then boosted the brightness and contrast to get what you see. The photos don't quite capture the blue glow that accompanies each big spark. The big sparks usually look like a thin, bright line surrounded by a pale blue cloud.

Photographing the sparks is a little tricky. I prefocused the camera with the lights on, then shut off the lights and opened the shutter for 30 seconds. IRIC, the camera was set to ISO 3200 and f4. While the shutter was open I walked over to the generator and moved my hand around near it and got the sparks to jump. That faint purple glow you can see surrounding some of the bigger sparks in the picture is there with every spark. It just doesn't show up very well in the photos.

I changed the top terminal from 11" to 14" diameter (still using Ikea Blanda salad bowls) which should allow the generator to hit 520 kV. It now discharges continuously from the top terminal to the brush on the bottom of the machine, so I slid one of the original 11" bowls down the tube to cover the bottom of the generator and this is what it did:

The distance from the top bowl bottom edge to the bottom bowl top edge is 550 mm.

I still have some optimizing to do- the sharp edges of the bowls have no insulation, so they tend to create corona discharge. I'll probably get another 14" bowl for the bottom, and maybe a longer piece of pipe...

Tuesday, September 19, 2017

I recently saw a couple pictures someone posted of two identical prints, made on the same machine with the same gcode file, one using very inexpensive Chinese import glass core GT2 belt and the other using a more expensive Gates belt. I was impressed by the reduced ringing in the print made using the Gates belt, so I decided to try this experiment for myself. Unfortunately, I've lost track of the link to those pictures.

I located a source and ordered 50 feet (the minimum quantity that Gates distributors will sell) of the Gates LL2MR09 belt (about $2 per ft. shipped).

One of the things that has always bother me about the Chinese belt was that there are exposed glass fibers along its edges. Well, the Gates belt has that, too. Both belts are neoprene with fiberglass core, both 2 mm pitch, and both 9 mm wide.

SKU

9396-0052

Part Number

LL2MR09

Profile

GT

Pitch

2 mm

Top Belt Width per strand (mm)

9

Tensile Cord

Fiberglass

Core Material

Chloroprene

Fabric Cover

Nylon

RMA Oil and Heat Resistant

Yes

Min Order Qty

50 ft

Max Cont Length (feet)

300 ft

Product Number

93960052

Weight

0.0200

Manufacturer

Gates

Brand

Gates

The Gates belt has nylon facing on the teeth which Gates says decreases wear and increases the life of the belt (and pulley?). Gates also specifies an operating temperature range of -54 to +85 C, so it should be fine inside a heated enclosure for printing ABS. I was unable to locate any operating temperature range spec for the Chinese belt.

The Chinese belt doesn't seem to have a nylon facing on the teeth, but I can see what appear to be the ends of threads embedded along the tooth surface under a microscope.

In the following photos the Gates belt is on the bottom and the generic Chinese belt in on the top.

The chinese belt, top, appears to have fibers embedded in the tooth surface, and the teeth look slightly larger than the Gates belt teeth. Glass fibers (brown) are visible on the edges of both belts.

Notice there are 17 glass cords in the chinese belt, top, and 20 in the Gates belt, bottom. Cord diameters appear to be about the same, spacing between the cords doesn't appear to be well controlled in either of them.

One minor difference is that the slicing of the Chinese belt doesn't seem to be particularly accurate. If you watch the edges of the belt as it moves on the printer, they seem to move up and down as if the top and bottom edges aren't parallel everywhere. The Gates belt doesn't do that.

I ran some print tests and there didn't appear to be any difference in print quality. Maybe the parts I printed weren't good for showing the differences. I'll be trying more prints and if I run into anything that reveals a big difference I'll post it here. It was enough effort to swap the belts that I don't expect to be doing it again without a really compelling reason.

This video shows one of the test prints- I turned up junction deviation to 0.2 (from 0.05) to induce ringing, and made the straight runs long enough to allow a peak speed of 250 mm/sec. I used the same gcode with the only difference between the prints being the belts in the XY stage. To my critical and microscope assisted eye, the prints are essentially identical. The ringing looks the same, the layer registration at the corners looks the same. Meh.

In the short term, these belts seem to perform pretty much the same, but print quality isn't the only criteria by which to judge a belt. If one belt outlasts the other and the drive pulleys used with one or the other last longer, one belt or the other might be better.